In digital spread spectrum (DSS) communication, a wide band carrier signal is modulated by a narrow band message signal. The wide-band carrier is typically generated by modulating a single frequency carrier using a pseudo-random noise (P/N) code sequence. The data rate at which a message is communicated is usually much lower than the P/N code symbol or “chip” rate. The ability of DSS to suppress interference is proportional to a ratio of the chip rate to data rate. In many applications, there are thousands of code chips per data bit.
At the receiver, a carrier replica is generated by reducing the DSS signal to baseband and multiplying it with a locally generated replica of the original narrow-band carrier using a local oscillator. If the frequency and phase of the carrier replica is the same as that of the received original narrow-band carrier, then the multiplier output signal will be the product of the bipolar P/N code and intended message. The P/N code is removed by multiplying the wide-band data stream with the locally generated replica of the P/N code that is time aligned with the received P/N code. This is the de-spreading process.
Generating the carrier replica with proper carrier frequency and phase and generating the P/N code replica at the proper rate and time offset is a complex problem. In many DSS communication systems, the necessary carrier frequency, carrier phase, and P/N code offset are not known a priori at the receiver, which tries different values until a large signal is observed at the data-filter output. This is termed the search or acquisition process, and a DSS signal is said to be acquired when the proper frequency, phase, and code offset have been determined. A receiver selects and detects a particular transmitted signal by choosing the appropriate P/N code and performing the acquisition search. In some cases the acquisition search must include examination of different P/N codes from a known list when the transmitting node is not known, as is the likely scenario in FIG. 1. When many different codes, code offsets and carrier frequencies must be examined and the SNR is low, the acquisition task can be both time and energy consuming.
The above constraints are more pronounced in a secure environment such as that depicted in FIG. 1 (detailed below), where a new node termed a hailing node 34 seeks to join an existing network while maintaining security for the joining node and those nodes already on the network. In addition, an established network requires a method of discovering the existence of another separate network that may have migrated into communication range, so that a cross-link can be established between the networks in order to form a larger network. This process of nodes “discovering” each other is termed herein node discovery, and is where DSS signal acquisition occurs. Typically, node discovery is done on channels separate from the primary data communication channels. Limited data exchange on the ‘discovery channel’ is preferable for network optimization. As a result, the discovery waveform must be flexible in the messages it carries and not be constrained to one specific message type or size.
The air interface should consist of a flexible and symmetric full-duplex or half-duplex link. The transmitting node or hailing node is that node that sends a discovery burst, essentially a message inquiring as to the presence of receiving nodes. Receiving nodes are the nodes that listen for that discovery burst. The receiving nodes are therefore target nodes, which may already have formed a network. These receiving nodes may become transmitting nodes when they send an acknowledgement back to the initiating new node. In this way, a new node that flies into range of an established network will transmit burst discovery messages on that transmitting node's transmit link. When a receiving node in the established network hears the discovery message on its receive link, it will respond via its transmit link which is the hailing node's receiving link. Subsequent handshaking can then be performed via the two nodes' transmit and receive links to bring the initiating new node into the network. The transmitting and receiving links may occupy separate time slots in a time division duplex (TDD) system, or may be separate frequency bands in a frequency division duplex (FDD) system.
An exemplary but non-limiting environment in which node discovery may be important is illustrated in perspective view at FIG. 1, a prior art arrangement of disparate nodes operating in a traffic data network and one hailing node seeking to join the traffic network. The nodes may be airborne as in aircraft or satellite; terrestrial as in autos, trucks, and trains; or waterborne as in ships and other surface watercraft. They may be stationary or mobile, fast or slow moving, as for example, communications between nodes in a building, an aircraft, and an auto. For additional flexibility, it is assumed that a hailing node 34 may not have a clock signal synchronized with the network prior to joining. The range 22 of the traffic data network is generally centered on a command node 24. The range 22 is included to show further advantages of the invention that may be exploited when network communications are geographically limited.
The command node is representative of the node that receives the discovery burst, and may be a true command node that controls access to the secure network (in that no other nodes receive and acknowledge discovery bursts) or it may represent any node already established within the network that receives a discovery burst (such as where all established nodes listen for discovery bursts). In FIG. 1, all nodes depicted as within the traffic network range 22 communicate on the traffic network, either through the command node 24 or directly with one another once granted network entry. The traffic network typically operates by directional antennas 24a, at least at the command node 24, to maximize the network range 22. This is because directional antennas typically enable a higher antenna gain and a higher tolerable path loss as compared to omni-directional antennas. Therefore, a range (not shown) of a discovery network that operates using omni-directional antennas 24b is somewhat less, at least in the prior art. The command node 24 maintains communication with stationary nodes 26, 28 or other moving nodes 30 already granted access to the network. When two nodes are aircraft, they may be closing or separating from one another at very high rates, rendering Doppler effects significant. When a hailing node 34 sends a discovery burst to locate and request entry into the traffic network, its signal is typically not received at the command node 24 until the hailing node is within the traffic network range 22. Since the hailing node 34 is not yet identified as authorized, this potentially puts communications within the network at risk, or alternatively unduly delays granting the hailing node 34 access to the network. Because access to the traffic network is obtained through the discovery protocol, that protocol must exhibit security features to prevent compromise of the traffic network.
Considering the issues apparent in light of FIG. 1, a good node discovery scheme for a highly secure communications network would therefore exhibit (a) high speed and reliability; (b) long range; (c) low probability of intercept (LPI) and low probability of detection (LPD) by unauthorized parties; (d) universal discovery and recognition among the various nodes; (e) asynchronous discovery; and (f) reliability for both stationary and fast-moving nodes.
Transmission bursts are normally divided into preamble and payload sections, payload carrying the substantive data and preamble to alert the intended recipient of the incoming payload. In a secure network, the preamble may be used to authenticate and grant access to the secure network, provided practical limitations as to real time signal processing for those functions can be accomplished with a high degree of reliability. As it is desirable to identify whether a hailing node 34 is to be allowed into the secure network as early as possible, directional antennas 24a (such as a rotatable dish antenna disposed in an aircraft nose cone) are typically used as they provide a longer range as compared to omni-directional antennas 24b (such as a fixed unipolar antenna protruding from an aircraft fuselage), all other factors being equal.
Two problems arise, one in establishing communications and one in maintaining them. First, the command node 24 may be scanning its directional antenna 24a in a quadrant that does not encompass the hailing node 34. Due to the high Doppler closure illustrated in FIG. 1, the hailing node 34 may be well within the (directional antenna 24a) range of the network when its discovery burst is first received at the command node 24. In a military environment, this puts friendly forces at increased risk unnecessarily for their lack of identification at the earliest possible moment. Second, when nodes are moving away form one another near the fringes of the omni-directional range (over which regular communications occur within the network), one fading node may switch to a directional antenna to extend the range of normal communications, but at a relatively high hardware cost of additional directional antennas for only a modest increase in range. There is an increasing need, in both business and military communications, for a highly secure network that extends over an area larger than that defined by a traditional range 22 (defined by a directional or omni-directional antenna). The present invention is directed to filling that need.